Promotional Effect of Molten Carbonates on Proton Conductivity and
Oxygen Reduction Reaction – An Experimental and Computational Study
Xiaolei Xiong
Solid oxide fuel cells (SOFCs) as a high-temperature fuel cell have the advantage
of fuel flexibility (capable of using existing hydrocarbon fuel) and high efficiency. The
recent research is aimed to lower the operating temperature to an intermediate
temperature (IT) range of 500 to 700oC, while maintaining a proper performance. This
Ph.D. research project investigates the promotional effects of alkaline carbonate
eutectics on the proton conductivity of proton conducting electrolytes and cathodic
ORR reactivity in SOFCs by both experimental and computational methods.
The MC-BZY composite electrolyte has been evaluated experimentally and
computationally to understand the enhanced conductivity and improved SOFC
performance. The ionic conductivity of the composite above 500oC increases with the
loading and type of MC. More importantly, the sample exhibited nearly a factor of two
higher conductivity in H2-containing atmosphere than in air. First-principles DFT
modeling further investigated proton transfer in a carbonate ion, lithium carbonate
crystal, lithium carbonate cluster and the interface of BaZrO3 and molten
carbonate. With the presence of carbonate ion, the energy barrier for proton migration
becomes as low as 0.332 eV, which agrees well with the activation energy of 0.33 eV
obtained by the experiment. The modeling indicates the reduction of energy barrier is
resulted from the change of rate-determining step from proton transfer between oxygen
atoms to proton rotation around oxygen atom.
Infiltration of MC into porous cathode can reduce the polarization of resistance
(Rp), i.e., enhance the oxygen reduction reaction (ORR) activity. The EIS analysis
shows that MC indeed has a beneficial effect on reducing Rp for different cathodes
including
Au,
La0.8Sr0.2MnO3-δ
(LSM), La0.6Sr0.4Co0.2Fe0.8O3-δ
(LSCF)
and
La2NiO4+δ (LNO). Specifically, the study on MC loading effect was carried out on
LSCF cathode. It shows that a higher loading makes a greater reduction on R p and the
degree of reduction is the same for 500 to 600oC. As the loading increases to 1.4 wt%,
the degree of Rp reduction tends to reach a limit. The lowest RP obtained at 2.8 wt% is
less than 2% that of the pristine sample within 500-650oC. First-principles DFT
modeling was further used to investigate the incorporation of oxygen into the molten
carbonate. The formation of CO52- in molten carbonate was considered as a
chemisorption of gas oxygen on the surface of MC infiltrated cathodes. After the
formation of CO52-, it reacts with another CO32- to form two CO42-, which is a ratelimiting step on potential energy surface. If the total reaction begins from the
incorporation of oxygen to CO32-, CO52- becomes an intermediate. After dissociation,
oxygen atoms migrate in molten carbonate, which is energetically favor by
intermolecular pathways. An oxygen atoms linkage is formed between carbonate ions,
which facilitates the oxygen migration between each carbonate ions.